1. Field of Invention
This invention relates to a new process for recovering commercially useful polyisocyanates or polyisocyanate mixtures from the distillation residues obtained in the production of tolylene diisocyanate.
Considerable quantities of relatively high molecular weight secondary products of uretdione, isocyanurate, carbodiimide, uretoneimine, urea and biuret structures are unavoidably formed in the industrial production of tolylene diisocyanate (TDI) by phosgenation of tolylenediamine. These secondary products accumulate in the form of a tar-like, non-distillable residue during separation of the crude reaction mixture by distillation. This residue adversely affects the economy of industrial tolylene diisocyanate production and is unsuitable for the typical applications of polyisocyanates (production of polyurethanes).
2. Brief Description of the Prior Art
These industrially unavoidable TDI distillation residues have been partly recycled by alkaline hydrolysis. However, only a small percentage of tolylenediamines is recovered in the course of an extremely slow hydrolysis process (U.S. Pat No. 3,128,310 or 3,331,876).
DE-OS 2,915,830 describes a process for obtaining TDI by treatment of the distillation residue in a fluidized bed at 140.degree. to 280.degree. C.; the residue left after separation of the isocyanate accumulating is of a dust-like consistency. This process and also the processes according to DD-PS 130,143 or DE-OS 2,452,804 are confined solely to the recovery of the TDI remaining in the distillation residue. One disadvantage common to all the above-mentioned processes is the accumulation of an unusable residue.
U.S. Pat. No. 3,634,361, DE-OS 2,123,183, DE-OS 2,333,150, U.S. Pat. No. 3,634,361 and DE-OS 2,423,594 describe several processes in which TDI distillation residues still preferably containing more than 20% by weight free NCO groups are dissolved in an organic solvent in the presence of monomeric diisocyanates, optionally at high temperatures. On account of its high solvent content, the mixture is limited in its potential uses. In addition, these methods have been inhibited in practice by the inadequate storability and standardizability of the solutions and by the sedimentation of insoluble constituents in storage.
Methods of utilizing the TDI distillation residue as such are also known and include, for example, the use of the ground residue as a filler (DE-OS 2,850,609). Although this method enables the distillation residue to be fully utilized, it is attended by the major disadvantage that the valuable TDI present in the distillation residue cannot be recovered.
DD-PS 257,827 describes a process in which, after mixing with diphenyl methane diisocyanate or its higher homologs with the distillation residue obtained in the production of TDI, the mixture is subjected to a distillation process in which TDI is recovered as distillate and a bottom product is recovered as a further isocyanate component. In this process, the TDI is not completely recovered which is a major disadvantage from the toxicological and physiological point of view. In addition, in the process according to this prior publication, either the distillation residue or the mixture to be distilled has to be pretreated with an acid acceptor or the bottom product accumulating has to be post-treated with an acid acceptor.
The problem addressed by the present invention is to provide a new process which would enable the TDI distillation residues obtained in the industrial production of TDI to be converted into a useful component containing isocyanate groups and the partly reversibly chemically bound TDI present in the distillation residue to be isolated as a further isocyanate component. In addition, the bottom distillation product accumulating would be stable in storage and further processable without any need for additives, for example acid acceptors, and the residual content of TDI in the bottom product would be below 200 ppm (by weight).
This problem has been solved by the process according to the invention which is described in detail in the following.